1. Field of the Invention
The invention relates to data communication utilizing the telephone voice communication network.
2. Description of the Prior Art
The telephone communication network has been utilized in the prior art not only for the usual voice communication function but also to transmit non-voice data. For example, equipment is commercially procurable that may be utilized with the conventional telephone handset for the transmission of documents over the telephone wires from one handset to another. These prior art devices operate within the standard telephone voice transmission frequency band and interfere with the use of the lines for voice communication while the non-voice data is being transmitted. Additionally, many of these devices require that the telephone handsets between which the data is being transmitted be in the "off-hook" state and connected through the telephone system switching matrices.
The conventional telephone communication network is also utilized for the transmission of data in data processing computer systems. Such signalling also utilizes frequencies within the standard telephone voice transmission frequency band normally precluding simultaneous use of the system for voice communication. In such systems, the terminals are interconnected with one another through the telephone company switching matrices with the telephone transmitters and receivers effectively in the "off-hook" state during the transmission of data. Additionally, such systems often utilize leased lines which add to the cost of the facilities. Data transmission over such leased lines is also effected within the standard telephone voice transmission frequency band of the system.
The conventional telephone communication network is also utilized for transmitting control signals, for example in a traffic control system or for the monitoring of instruments from a central location, such as is often required by utility companies. All such installations utililze the telephone communication network at frequencies within the standard telephone voice transmission frequency band of the telephone system. When data is transmitted over such installations, normal telephone voice communication is usually precluded and, conversely, when it is desired to use the lines for voice communication, data transmission and monitoring is terminated. Such systems often utilize leased telephone lines for the data transmission function which adds to the cost of the system.